1. Field of the Invention
This invention pertains to the field of the extraction of nickel from aqueous solutions thereof by liquid-liquid extraction.
2. Description of the Prior Art
Benzophenone oxime and 8-hydroxyquinoline derivatives are known agents for the extraction of nickel from aqueous ammoniacal solutions or solutions containing ammonium. The use of these extraction agents, among others, is reported in Chemical Abstracts, Volume 82, No. 26, Report 173 986.
The use of these agents requires the presence of ammonium ions which remain in the refined product. In processes without circulation in the aqueous stage, this proves disadvantageous because it causes environmental pollution and is expensive.
Another group of extraction agents consists of materials, such as, phosphoric acid esters of which the di-2-ethylhexyl phosphoric acid is especially important and naphthenic acid which is especially suitable to extract nickel from ammonium-free solutions in the pH range of 4 to 8. The use of these extraction means, among others, is described in the Journal of Inorganic Nuclear Chemistry, 1974, Vol. 36, page 189 to 192, Pergamon Press, printed in Great Britain.
The use of di-2-ethylhexyl phosphoric acid is disadvantageous since in the extraction, the alkali salts of the acid must be formed or the alkali salts are formed at increased pH values (pH&gt;4) and therefore, a high content of alkali is obtained which is very difficult to displace by nickel. The extraction is made more difficult when the aqueous stage already contains a significant concentration of sodium ions from preceding stages of use of the solution. This can be immediately concluded from the law of mass action shown in the following equation: EQU 2 R-Na+Ni.sup.++ .revreaction.R.sub.2 -Ni+2 Na.sup.+.
The naphthenic acid acts differently since it extracts nickel only at the relatively high pH values of 7 to 8, however the alkali ions are completely displaced by nickel. In this method of employing the naphthenic acid as an extraction agent, the especially slow extraction speed, i.e., greater than 30 minutes, has a continuous effect until the nickel is separated. However, this slow extraction speed does not provide a practical technical method. The reason for this is seen in the fact that the nickel, at pH 7 to 8, is already partially separated as nickel hydroxide.
Thus, a method for obtaining nickel from solutions having especially large contents of alkali salts in the organic stage and, therefore, in the nickel strip eluate resulting therefrom was desired. The method of choice was the liquid-liquid extraction of nickel at a speed suitable for practical processing and, at the same time, possessing a distribution coefficient as high as possible to insure a practically complete displacement of the alkali content.